6 yr old onion seeds didn't sprout...
How long do seeds last?
I've given up on my 6 year old onion seeds. :-( They didn't sprout this year. Time to throw out the package. I think also last year they also didn't sprout. It may have been the previous year (4 years old) that was the last time they came up. And that year, they took an extra week to come up.
Onions seeds are one of the more short-lived types. Its often recommended you only use them the season they are packaged for. Other short-lived seeds include corn, leek, onion, parsnip, and spinach. Hmm, I haven't bought spinach in ages. Mine have saved very well. But I agree with putting parsnip in this list and I'd add carrots here. I have some 5 year old popcorn seeds I was thinking of using again this year. I'll try a test germination on paper towels to see if there's any life left in them.
Long-lived seeds include beets, all brassicas (broccoli, Brussels sprouts, cauliflower, cabbage, collards, kohlrabi), chicory (endive, escarole, radicchio), cucumber, kale, lettuce, melons, mustard, peppers, radish, rutabaga, sunflower, tomato, and turnip. Long-lived means 5-10 years if kept cool and dry.